Such amplifiers are generally well known. They make it possible to boost the braking force supplied by the brake pedal and supply this boosted force to the master cylinder Particular reference will be made here to the amplifier described in Patent EP-0 509 866. The amplifier comprises a cavity in which a movable wall separates a vacuum chamber that is in permanent communication with a vacuum source and a working chamber. The amplifier comprises a shutter (control valve) connected to the brake pedal and able to bear against a seat contiguous to an intermediate chamber in permanent communication with the vacuum chamber.
When it is remote from the seat, the shutter places the intermediate chamber in communication with the working chamber such that the working chamber is subjected to the partial vacuum prevailing in the vacuum chamber. This is the situation with the amplifier at rest. When a braking action is required, the shutter isolates the intermediate chamber from the working chamber and then gradually places the later in communication with atmospheric air. The pressure thus increases in the working chamber, which causes the movable wall connected to the master cylinder to move in the cavity. When the braking command is broken off, the shutter retreats in order to move away from its seat and place the intermediate chamber once more in communication with the working chamber.
However, at this stage, since a relatively low pressure prevails in the intermediate chamber and since a higher pressure prevails in the working chamber, the shutter is often pressed against its seat by the pressure difference. The plunger that actuates the shutter for this purpose must therefore supply a substantial force to detach the shutter from its seat.
In addition, the load, and therefore the stiffness, of the spring associated with the shutter must be relatively high. These two factors generate hysteresis between the outward curve and the return curve connecting the output force of the amplifier to the input force. This hysteresis is manifested in practice by the driver experiencing a dissimilarity in feel between applying and releasing the brake pedal.